This proposal seeks to understand the processes driving the increasing prevalence of allergic immune diseases in developing countries. This type of immunity, here termed type 2 immunity, commonly occurs during intestinal helminth infection, and likely underlies the evolution of this host response. We and others have noted the association of cellular constituents of allergic immunity, including eosinophils and alternatively activated macrophages, in mouse visceral adipose tissues, where these cells are required to sustain normal metabolic homeostasis. As shown here, these cell types are maintained in adipose tissues by constitutive activation of innate helper type 2 (ILC2) cells, a novel type of innate lymphoid cell simultaneously reported by this and two other laboratories. This grant seeks to answer the hypothesis that ILC2 cells link mucosal integrity with metabolic homeostasis by attenuating lung and intestinal mucosal inflammation while sustaining systemic energy demands. The grant seeks to pursue this hypothesis in 3 Specific Aims: 1. To establish the role of ILC2 cells in immune and metabolic homeostasis associated with allergic inflammatory challenge; 2. To establish the role of ILC2 cells in immune and metabolic homeostasis in animals with genetic deficiencies associated with mucosal inflammation of the lung and intestinal mucosa; and 3. To assess the effects of intestinal parasitic infection on the microbiome and the role of type 2 cytokines in mediating these effects. The novel technical approach involves the use of unique lines of mice established in my laboratory with knockin fluorescent markers containing embedded Cre elements that facilitate cell function-marking, fate-mapping and cell-specific deletion based on function with a high degree of precision. Our approach will open up new lines of investigation in understanding the evolutionary role of ILC2 cells and possibly yield insights into the relationship of allergic immunity, mucosal integrity and systemic energy homeostasis.